Why Clean, No Clean Solder Paste?

PCBAssemblyEngineering

The point of no-clean solder pastes was to eliminate the need to clean circuit board
assemblies. So why do most assemblers still clean?

Most manufacturers have developed and fine-tuned their assembly processes to run
no-clean paste and would rather run all production with one process, and only clean
those which require it, than to adapt processes to accommodate multiple types of
paste.

Since no-cleans are designed so they don’t require cleaning, any remaining flux
residue left behind by no-clean pastes are ironically more difficult to remove than
other fluxes. These trace residues are not designed to easily wash off which creates
a problem if you need to clean them. No-clean flux is really “low-residue” solder
paste. Post reflow, it leaves trace amounts of noncorrosive, resinous residue on
or around solder joints, varying in color from transparent to amber. The lower the
solids content (resins, gelling agents and activators) in the paste, the lower the
residue left on the board.

Assemblers first began cleaning no-clean fluxes due to problems found at in-circuit
testing. Residues from early, no-clean pastes were tacky and test pins would not
always penetrate through to make good electrical contact with test pads. Further,
the residue would build up on pins affecting accuracy, and necessitating maintenance.
Since recent formulations have eliminated this problem, no-clean fluxes are now
cleaned.

to prevent circuit malfunctions with clock speeds over 1 gigahertz.

to improve adhesion of conformal coating and under fill materials (flux residues
are hydroscopic, so it’s possible for small pockets of steam or gas to be generated
during curing affecting adhesion and integrity).

to remove solder balls and improve overall cosmetic appearance.

No clean pastes can’t be cleaned with just water, rather a solvent or solution of
water and saponifier is required. The choice of flux chemistry (low or high solids,
hard or soft residues, etc.) determines the most effective cleaning process to employ.
Fluxes containing halides leave more residue, but are easier to clean using mild
cleaning agents and short wash times. Alternatively, halide-free, no-cleans produce
less residue, but are more difficult to clean. Moreover, exposing no-clean pastes
to excessively high temperatures causes residue to harden and solder joints to become
dull due to oxidation. For optimum results, just like with soldering, it’s best
to follow the paste manufacturer’s recommendations.

Lead-free, no-clean paste can be difficult to clean. Reflow temperatures for lead-free
solder are upwards of 240C making residues hard to get off. So process engineers,
are advised to resolve cleaning issues early in the design process before solder
paste, coatings, and other materials are designed in. If you can select a paste
that’s easier to clean, it can make life much easier down the road for you, the
assembler, and your customer.